In the field of cellular telecommunications, efforts have recently been directed towards developing Code Division Multiple Access (CDMA) type systems In a CDMA type system multiple users, each using a channel identified by a uniquely assigned digital code, simultaneously communicate with the system while sharing the same wideband frequency spectrum.
CDMA provides several advantages over conventional frequency division multiple access (FDMA) or time division multiple access (TDMA) systems. In FDMA systems users are assigned a unique frequency for each of mobile to base (uplink or reverse link) and base to mobile (downlink or forward link) communications In TDMA systems, users are each assigned a unique frequency, for the uplink and downlink, and a unique time period in which to transmit or receive on that frequency These FDMA and TDMA systems require planning for allocation of channel frequencies and/or time periods on these frequencies to mobile stations and base stations. In a CDMA system, however, frequency and time period assignment planning for mobile stations and the base stations of cells is not necessary, as in FDMA and TDMA systems, because all CDMA base stations share the entire downlink frequency spectrum, and all mobiles share the entire uplink frequency spectrum The fact that the wideband frequency spectrum is shared by all uplink or downlink users in CDMA also increases capacity, since the number of users that can be multiplexed simultaneously is only limited by the number of digital codes available to identify the unique communications channels of the system, and by the total interference caused by the other users sharing the same spectrum, and not by the number of radio frequency channels available. Additionally, since the energy of the transmitted signals are spread over the wide band uplink or downlink frequency band, selective frequency fading does not affect the entire CDMA signal. Path diversity is also provided in a CDMA system. If multiple propagation paths exist, they can be separated as long as the differences in paths delays exceeds 1/BW, where BW equals the bandwidth of the transmission link.
In a CDMA cellular system the transmission power levels of mobile stations become important. In CDMA, the signals from many different mobile stations are received at the same frequency simultaneously at a base station. Because of the nature of CDMA demodulation, it is necessary that the signal received at the base station from each mobile station be as close as possible to a single level so that the signal from one mobile station does not overwhelm the signal from another mobile station (near-far problem). In a CDMA cellular system a power control process may be used to control each mobile station's transmission power level so that the signal level received at the base station from each mobile station is as close as possible to a single predetermined level. Additionally, the power control process may also be used to assure that the received signal levels at the base station are of an adequate level, so that calls are not dropped One example of a CDMA mobile station power control scheme is the power control used by systems specified in the Telecommunications Industry Association/ Electronic Industries Association (TIA/EIA) IS-95 standard Another related CDMA mobile station power control scheme is the power control used by systems specified in the ANSI-008 standard, which is the personal communications systems (PCS) 1900 Mhz version of IS-95.
In the IS-95 power control process, a mobile station first adjusts its transmission power level using an access channel assigned to a base station through which the mobile station is attempting to gain access to the system. To gain access the mobile station follows an open loop power control process that involves transmitting access probe transmissions at a relatively low power level on the access channel and gradually increasing the level of subsequent access probe transmissions in access probe correction increments set by the system, until a response is obtained from the system and the mobile station gains access to the system The transmission power of each access probe transmission on the access channel is given by the equation: ##EQU1##
The values NOM.sub.-- PWR and INIT.sub.-- PWR are system parameters having values assigned by the system. The mean input power is the power level of the reverse link access channel signal as received at the mobile station.
The ANSI-008 standard equation is similar, but because of the frequency difference, has a constant equal to 76 instead of 73, and also includes an additional value that is added to increase the range of NOM.sub.-- PWR, the additional value being defined as 16*NOM.sub.-- PWR.sub.-- EXT, which is a system parameter. IS-95 could also be modified to include a similar value.
Once the mobile station gains access to the system, it waits in an idle mode until a call is initiated from either the mobile station to the base station, or from the base station to the mobile station. A reverse traffic channel and a forward traffic channel are then assigned for the call. When transmitting on the IS-95 reverse traffic channel, the mobile station initializes at the level set on the access channel by the open loop process defined by equation (1). Once the reverse traffic channel transmission power level is initialized and the call begins, the system and mobile station then also begin a closed loop power control process The closed loop power control process allows the signal level received at the base station from each mobile station transmitting on a reverse traffic channel to be set as close as possible to a single predetermined level. In the closed loop power control process the base station transmits closed loop power control corrections in the form of power control bits to the mobile station in a power control subchannel that is included in the forward traffic channel. A single power control bit is transmitted in the power control subchannel every 125 ms. A "one" bit transmitted in the power control subchannel indicates that the mobile station should increase its transmission power 1 db, while a "zero" bit indicates that the mobile station should decrease its transmission power 1 db. Each time a valid control bit is received at the mobile station in the power control subchannel, the mobile station adjusts its output power level up or down in an increment of 1 db. The mobile station is capable of adjusting the transmission power within a range of .+-.24 db around the level set by the open loop power control process on the access channel and, as the call is ongoing, the transmission power of the mobile station on the reverse traffic channel is adjusted so that a desired power level is reached and maintained The mobile station also simultaneously continues the open loop power control process, this time using the forward traffic channel. In the open loop process the mean input power received on the reverse traffic channel is the determining value. When involved in the call, the inputs that effect changes in the mobile station's transmission power are the mean input power as received from the base station on the forward traffic channel and, the closed loop power corrections indicated by the power control bits received on the forward power control channel. During a call, the mobile station output power level on the reverse traffic channel is given by the equation: ##EQU2##
As for equation 1, the ANSI-008 standard equation is similar, but because of the frequency difference, has a constant equal to 76 instead of 73, and also includes an additional value that is added to increase the range of NOM.sub.-- PWR, the additional value defined as 16*NOM.sub.-- PWR.sub.-- EXT, which is a system parameter.
A handoff of the call from one base station to another base station may occur as the mobile station moves throughout the system In a CDMA system two types of handoff are possible A soft handoff occurs when the mobile station commences communications with the new base station without interrupting communications with the old base station. Soft handoff can only be used between CDMA traffic channels having identical channel frequency assignments. A CDMA to CDMA hard handoff occurs when the mobile station is transitioned between two disjoint sets of base stations, different frequency assignments, or different traffic channel frame offsets.
During soft handoff of a call, the system continues to use the open loop power control process to adjust the transmission power of the mobile station on the reverse traffic channel as the mobile switches to the traffic channels of the new base station. During CDMA to CDMA hard handoff a value of a nominal power (NOM.sub.-- PWR) setting for the target cell is transmitted to the mobile station in a handoff message. The nominal power(NOM.sub.-- PWR) setting is then used to set an initial power transmission level on the traffic channel of the new base station. When the mobile station begins transmitting on the new reverse traffic channel, open and closed loop power control then take over so that the transmissions continue at the correct level.
In a CDMA system having cells of different sizes and base stations with different size transmitters, the base station in a particular cell may have a different desired received signal strength for signals received from mobile stations, as compared to another cell's base station's desired received signal strength for signals received from mobile stations If this difference exists between neighboring cells, a problem could occur during handoff of a call between the neighboring cells. If the handoff is from a higher power cell to a lower power cell, there could be a potential near-far problem among the mobile stations in the new cell. The mobile station involved in the call handoff may initially be at a power level much greater than other mobile stations in the new cell. The signal from the mobile station involved in the call handoff may then overwhelm the signals from the other mobile stations at the base station causing a near far problem If the handoff is from a lower power cell to a higher power cell, there could be a potential dropped call problem. The mobile station involved in the call handoff may initially be at a power level much lower than other mobile stations in the new cell. The signal strength received at the new base station from the mobile station in call handoff may not be adequate to maintain the connection on the reverse link, and a dropped call could occur in this situation.
For the IS-95 type hard handoff, the power control process used may not be adequate to avoid problems in call handoff between cells of different sizes Again, because handoff occurs at the threshold where the received signal level on the measurement channel of the current base station is slightly greater than the received signal level on the measurement channel of the new base station, the variable of the mean input power used in equations (1) will not cause a significant change in mobile station transmission power. In IS-95 the transmission power level of the mobile station is adjusted at hard handoff by the new base station transmitting a new NOM.sub.-- PWR value to the mobile station. The NOM.sub.-- PWR value is typically an estimated value, set to adjust the mobile station transmission power level to an appropriate initial level in the target cell. The new NOM.sub.-- PWR value is then used in the mobile station to calculate a new mean output power, as given by equation (1) above for the initial transmission of the mobile station on the reverse traffic channel after handoff. In this process the range of the value of NOM.sub.-- PWR is from -8 dB to 7 dB. In the ANSI-008 standard the value 16* NOM.sub.-- PWR.sub.-- EXT is used along with NOM.sub.-- PWR in equation 1, so that the range is from -24 dB to 7 dB. In either system, if the mobile station transmission power level is needed to be adjusted beyond what the range allows, a dropped call or near far problem could occur.